The only reason some rare variations Speed six brings 800 or 1250 is because of their rarity. If Ruger starts making them again the price would fall, and be close to what a 2 ¾ inch .357 sells for.
The Speed six is one of my favorite SS guns and I have some in 2 ¾". One in .357, one in .38, and one in 9mm. I would not sell the 9mm for $1,000 as it would take that or more to replace it. The .357 I value at about $600 and the .38 at about $550. All are in very good condition. Currently have what I want so I am neither a buyer or seller, unless of course I spot one at a ridiculously low price, or someone makes me a ridiculously high offer to buy.
I think depending on condition the SS 2 ¾ Speed six in .357 will sell for from $475 to $700 depending on condition (ranging from scruffy to NIB).
I think depending on condition the Blued Speed 2 ¾ six in .357 will sell for from $425 to $675 depending on condition (ranging from scruffy to NIB).
I think depending on condition the SS 2 ¾ Speed six in .9mm will sell for from $900 to $1250 depending on condition (ranging from scruffy to NIB).
Some rare issues will bring more. I don't have a clue about the 3', but as rare as they are they would have to bring a real premium over the 2 ¾ in. This is a collector premium, as I really like the 2 ¾" and I would not pay a penny more for that extra ¼ inch, but then I buy the Speed Six as a self defense gun, not as something to collect.
In general on the Six series, the 2 ¾ brings a premium of $100 to $150 over the 4 inch. The SS brings a premium of about $50 to $75 over the blued.
For my money the six series are the most under priced revolver available today, and they are still under valued due to the fact that Ruger sold so very many of them and they are almost indestructible. My favorites Six Series Revolvers are the 2 ¾ in Speed Six and the 4 inch Security Six. Both truly great revolvers.
Unlike most guns the Ruger Six Series seem to get better the more you shoot and use them. The point when other guns are starting to have problems the Six Series Revolvers are just getting broken in good.